Fluid energy mills incorporating a vortex propelled by supersonic jet nozzles, referred to as a Micronizer™, are used to reduce the particle size of materials by particle-on-particle impact without the use of moving parts. The mill generally has a grinding chamber with nozzles arranged peripherally tangent to an imaginary circle within the grinding chamber. Compressed gas such as air, steam, nitrogen, etc. is introduced through the nozzles and creates a swirling vortex of gas which travels at high speed around the chamber, at decreasing radii, until the gas exits at an outlet located at the center of the grinding chamber. Feed material is introduced to the grinding chamber as far outside of the grinding nozzle tangent circle as possible to maximize grinding time. The material becomes entrained in the vortex where the rotation generates high-speed particle-on-particle collisions and collisions with the grinding chamber walls creating increasingly smaller particles. Heavier particles stay in the vortex the longest, held there by centrifugal force, until they are light enough to move with the vortex around the chamber and exit with the stream at the outlet. Such mills are capable of producing particle sizes down to the sub-micron range without the introduction of heat common to other forms of particle size reduction.